1. Technical Field
Plural MRI RF transmit coils are actively decoupled from one another. This is especially useful for Sensitivity Encoding for Fast MRI (SENSE).
2. Related Art
Magnetic resonance imaging (MRI) utilizes a strong, uniform and static magnetic field B0 to polarize the magnetic moment of nuclear spins in a human body or other objects. The magnetically polarized nuclear spins generate a net magnetization which points in the direction of magnetic field B0. However, this produces no useful information unless disturbed by some excitation.
The generation of a nuclear magnetic resonance (NMR) signal for MRI data acquisition is achieved by exciting the nuclear magnetic moments with a uniform radiofrequency (RF) magnetic field B1 at the Larmor frequency for those nuclei that are to be excited. An RF transmit coil emits the B1 field in an imaging region of interest (ROI) when driven by a computer-controlled RF transmit unit. The most common type of RF transmit coil is a birdcage body coil.
FIG. 1 shows a traditional RF transmit coil 13 and a computer-controlled RF transmit unit including an RF signal generator 10, a magnitude/phase controller 11 and an RF Power amplifier 12. Typically, the transmit unit includes only one RF power amplifier. This RF power amplifier may possess a huge power rating (e.g., exceeding 30 KW for a 3T MRI system).
During excitation by the RF transmit coil, the nuclei that are at their Larmor resonance absorb magnetic energy, and their respective magnetic moments (spins) precess around and are rotated away from the direction of magnetic field B0. After excitation, the precessing angularly displaced magnetic moments undergo free induction decay back to alignment with B0. During this free induction decay, the nuclei emit their absorbed energy as RF signals as they return to steady state condition. An RF receiving coil positioned in the vicinity of the excited nuclei detects an RF NMR signal. The NMR signal is represented as an electromotive force (voltage) in the receiving RF coil that has been induced by a flux change over some time period due to the relaxation of precessing magnetic moments. This signal provides contrast information for an image.
The receive RF coil may comprise either the transmit coil itself using a Transmit/Receive (T/R) switch or an independent receive-only RF coil. The NMR signal is spatially modulated for producing magnetic resonance images by utilizing additional pulsed magnetic gradient fields, which are generated by gradient coils which vary linearly with respect to spatial coordinates in the imaging volume and thus serve to spatially phase encode the RF NMR signals. Gradient field(s) can also be used during RF excitation to selectively excite a specific sub-volume (e.g., a slice) of the ROI.
It is desirable in MRI for the RF excitation and reception to be spatially uniform in the ROI imaging volume for better image uniformity. In a typical MRI system, a whole-body volume RF transmit coil usually produces the best excitation field homogeneity. The whole-body volume coil is the largest RF transmit coil in the MRI system. The large size of this transmit coil, however, produces a lower signal-to-noise ratio (SNR) if it is also used for reception, mainly because of its greater distance from the signal-generating tissues being imaged. Since a high SNR is highly desirable in MRI, dedicated coils are often used for reception to enhance the SNR from the ROI.
In practice, a well-designed dedicated RF transmit coil should possess the following functional properties: a high SNR, good uniformity, a high unloaded quality factor (Q) of the resonant circuit, and a high ratio of the unloaded to loaded Q factors. In addition, the mechanical design of the coil should facilitate patient handling and comfort and provide safety protection between the patient and RF transmit coil electrical conductors.
Quadrature reception provides another way to increase the SNR. In quadrature reception, two independent (i.e., decoupled) individual RF receiving coils detect NMR signals in two orthogonal modes, which may be associated with planes transverse and perpendicular to the main magnetic field B0. The two receiving coils cover the same volume of interest. With quadrature reception, the SNR may be increased by up to √{square root over (2)} over that of individual non-QD coils.
A linear surface coil array technique in MRI may cover a large field-of-view (FOV), while maintaining the SNR characteristics of a small and conformal coil. A linear surface coil array technique may be used to image an entire human spine (see U.S. Pat. No. 4,825,162). Other linear surface array coils have been used for C.L. spine imaging (see U.S. Pat. No. 5,198,768). These devices may comprise an array of planar linear surface coil elements. However, these coil systems do not work well for imaging deep tissues, such as the blood vessels in the lower abdomen, due to a drop-off in sensitivity at positions not so close to the surface coil.
Quadrature phased array coils have been utilized to image the lower extremities (see U.S. Pat. Nos. 5,430,378 and 5,548,218). Quadrature phased arrays may image the lower extremities by using two orthogonal linear coil arrays: (i) six planar loop coil elements placed in the horizontal plane and underneath the patient and (ii) six planar loop coil elements placed in the vertical plane and in between the patient's legs. Each linear coil array functions in a similar way as described in U.S. Pat. No. 4,825,162. A second quadrature phased array coil has been designed to image blood vessels from the pelvis down. This device also comprises two orthogonal linear coil arrays extending in the patient's head-to-toe direction: a planar array of loop coil elements laterally and centrally located on top of a second array of butterfly (also referred to as “figure-8”) coil elements. The loop coils are placed immediately underneath the patient and the butterfly coils are wrapped around the patient. Again, each linear coil array typically functions in a way similar to that described in U.S. Pat. No. 4,825,162.
Gradient coils are routinely used in MRI to provide phase-encoding information to RF MRI signals. To obtain an image, all data points in a so-called “k-space” (i.e., frequency space) are typically collected. Recently, there have been developments where some of the data points in k-space are intentionally skipped. Time intrinsic sensitivity information of RF receive coils are used to phase-encode information for the skipped data points. These operations occur simultaneously and are thus referred to as parallel imaging. Collecting multiple data points simultaneously requires less time to acquire the same amount of data when compared with conventional gradient-only phase-encoding. Time savings may be used to reduce total imaging time, which is particularly helpful for those applications in which cardiac or respiratory motions in imaged tissues cause concern. The time savings may alternatively be used to collect more data to achieve better resolution or SNR. SiMultaneous Acquisition of Spatial Harmonics, SMASH, (U.S. Pat. No. 5,910,728) and “Simultaneous Acquisition of Spatial Harmonics (SMASH): Fast Imaging with Radiofrequency Coil Arrays,” Daniel K. Sodickson and Warren J. Manning, Magnetic Resonance in Medicine 38:591-603 (1997), (both of which are incorporated herein by reference) and “SENSE: Sensitivity Encoding for Fast MRI,” Klaas P. Pruessmann, et. al., Magnetic Resonance in Medicine 42: 952-962 (1999), (also incorporated herein by reference), disclose two techniques of parallel imaging. The SMASH technique takes advantage of parallel imaging by skipping phase encode lines that yield a reduction in a Field-of-View (FOV) in the phase-encoding direction and uses spatial harmonics produced by coil arrays to fill in the missing data points in k-space. The SENSE technique, on the other hand, utilizes an aliased image obtained as a result of skipping some k-space data points and then unfolds the aliased image in x-space (i.e., real space) by using individual RF transmit coil sensitivity information in the RF transmit coil arrays.
The SENSE and SMASH techniques, or a hybrid approach of both, demand new design requirements in RF transmit coil design. In SMASH, the primary criterion for the array is that it be capable of generating sinusoids whose wavelengths are on the order of the FOV. This is how the target FOV along the phase encoding direction for the array is determined. Conventional array designs can incorporate element and array dimensions that will give an optimal SNR for the object of interest. In addition, users of conventional arrays are free to choose practically any FOV, as long as severe aliasing artifacts are not a problem. In contrast, when using SMASH, the size of the array determines the approximate range of FOVs that can be used in imaging. This range determines the approximate element dimensions, assuming complete coverage of the FOV is desired, as in most cases.
The SENSE method is based upon the sensitivity of an RF receive coil generally having a phase-encoding effect complementary to those achieved by linear field gradients. For SENSE imaging, the elements of a coil array may be smaller than for common/conventional phased-array coils, thereby resulting in a trade-off between basic noise and geometry factor (referred to as g-factor). Designs where adjacent coil elements are not overlapped have been suggested for a net gain in SNR due to the improved g-factor when using
  SENSE  ⁢          ⁢      (                  SNR        ⁡                  (          SENSE          )                    =                        SNR          ⁡                      (            Full            )                                    g          ⁢                      R                                )  where R is a reduction factor.
A Transmit-SENSE method has been suggested to address specific absorption rate (SAR) issues as the static main magnetic field B0 becomes larger (e.g., greater than 3T—see U. Katscher, et al., Magnetic Resonance in Medicine 49: 144-150 (2003)). To illustrate the idea of Transmit-SENSE, multiple RF transmit coils are placed around the human body being imaged. Each RF transmit coil can deliver its own B1 field with different phase and magnitude. The resultant B1 field is the sum of the fields from all the RF transmit coils. One of the requirements to implement Transmit-SENSE successfully is that all the RF transmit coils be mutually decoupled from each other. However, achieving an adequate level of decoupling among all the transmit coils becomes challenging as the total number of transmit coils increases. Achieving adequate decoupling between the transmit coils is a problem because conventional decoupling techniques such as a low input-impedance preamp decoupling method used in RF receive array coils are difficult to realize in RF transmit coils. That is, known methods used to decouple RF receive coils are not readily transferable to decouple RF transmit coils. One of the decoupling methods proposed by K. N. Kurpad, et al., “A Parallel Transmit Volume Coil with Independent Control of Currents on the Array Elements,” Proceedings of International Society for Magnetic Resonance in Medicine 13 (2005), uses a high impedance current source to drive the RF transmit coil. However, implementing this method may not be economical. For example, the power source used in this method is not an industry standard 50 Ohm impedance power RF amplifier.